Rainwater Harvesting Volume
Calculate annual harvestable rainwater volume from your roof based on catchment area, rainfall, runoff coefficient, and first-flush losses.
Results
What is it?
Rainwater harvesting collects precipitation from rooftops and stores it for non-potable or potable use (with proper filtration). The harvestable volume depends on catchment area, local rainfall, roof surface type (affecting runoff), and first-flush diversion (the initial rainfall washes off pollutants and is discarded). Average household water use is 100-150 L/person/day.
How to use
1. Enter your roof catchment area in square meters (use the horizontal projected area, not the actual sloped area). 2. Enter your local annual rainfall in mm (available from weather services or local water authority). 3. Select your roof type to set the runoff coefficient. 4. Enter the first-flush loss percentage (10% is a common default).
Example scenario
A 100 m2 metal roof in a region with 800 mm/year rainfall. Gross = 100 x 0.8 x 0.90 = 72 m3. With 10% first-flush loss: net = 72 x 0.9 = 64.8 m3 = 64,800 L/year. That is 5,400 L/month or 177 L/day — enough to supply water for garden irrigation and toilet flushing.
Pro tip
Size your storage tank to capture the most rainfall from wet-season peaks. A rule of thumb is to size the tank for 1-2 months of supply. For potable use, add UV sterilisation and carbon filtration. Many municipalities offer rebates for rainwater tanks — check with your local water authority before installing.